1. Field of the Invention
The present invention relates to an electrode material for an electrochemical capacitor, an electrochemical capacitor comprising an electrode containing such an electrode material, and a method for the production of such an electrode material.
2. Prior Art
Electric double layer capacitors utilize the capacitance of an electric double layer formed at an interface between a solid and a solution. In general, an electric double layer capacitor comprises a pair of polarizable electrodes (positive and negative electrodes), an electrolyte impregnated in the electrodes, a porous separator which contains an electrolyte impregnated therein, is ion-permeable and has electrical insulation properties so that it separates the electrodes to prevent the formation of a short circuit between them, and collectors connected to the respective electrodes. For example, in the case of a coin-type capacitor, a pair of electrodes, a separator interposed between the electrodes, and an electrolyte are stored in a metal case (can), and the case is sealed with a metal lid through an electrically insulating gasket to prevent the leakage of the electrolyte.
Such electric double layer capacitors have intermediate properties between the properties of cells and those of electrolytic capacitors, and have a large capacitance with a small volume. Thus, in these years, electric double layer capacitors attract attentions as the backup sources of small-sized electric devices, etc.
Electric double layer capacitors comprise polarizable electrodes as positive and negative electrodes, as described above. Hitherto, activated carbon having a large surface area is used as the material of such polarizable electrodes, since it has a large specific surface area and carries a large amount of electric charge on its surface, and thus the capacitance of electric double layer capacitors increases. In general, electrolytes comprise a medium having a high dielectric constant such as water or carbonates (e.g. propylene carbonate, etc.) to dissolve solutes in a high concentration.
However, the maximum specific surface area of activated carbon is practically about 3000 m2/g. Thus, the capacitance per unit volume of electric double layer capacitors comprising electrodes containing activated carbon almost reaches its limit. Therefore, electrochemical capacitors are proposed, which comprise electrodes containing rutile-type ruthenium oxide, amorphous hydrated ruthenium oxide, or their hydrogenated products (e.g. RuO2.xH2O, etc.), which have a larger capacitance than activated carbon. However, electrochemical capacitors having electrodes containing such ruthenium oxides have a problem such that material costs are high since ruthenium is a rare material, although they have a ten to fifty times larger capacitance than those having electrodes containing activated carbon.
One object of the present invention is to provide a novel electrode material to be used to produce electrodes of electrochemical capacitors, which can provide capacitors at a low cost and achieve a high capacitance.
Another object of the present invention is to provide a method for the production of such an electrode material.
A further object of the present invention is to provide a low-cost electrochemical capacitor having a large capacitance using such an electrode material.
Accordingly, the present invention provides an electrode material for an electrochemical capacitor, comprising at least one titanium oxide compound selected from the group consisting of titanium oxide, hydrated titanium oxide and their hydrogenated products, and at least one oxidizable and reducible metal element contained in the titanium oxide compound.
Such an electrode material has a large capacity, for example, 300 coulombs per one cm3 of the material.
Also, the present invention provides an electrochemical capacitor comprising a pair of electrodes at least one of which comprises an electrode material according to the present invention, a porous separator interposed between the pair of electrodes, and an electrolyte. Preferably, the electrodes, separator and electrolyte are enclosed in a case.
In addition, the present invention provides a method for the production of an electrode material for an electrochemical capacitor comprising the steps of:
dipping a substrate containing titanium in a solution of salt of at least one oxidizable and reducible metal,
pulling out the substrate from the solution and drying it, and
calcining the dried substrate.
Furthermore, the present invention provides a method for the production of an electrode material for an electrochemical capacitor comprising the steps of:
forming fine particles of an electrode material from at least one compound selected from the group consisting of salts, alkoxides and chelates of titanium, and at least one compound selected from the group consisting of salts, alkoxides and chelates of oxidizable and reducible metals, by an evaporation method, a co-precipitation method or a sol-gel method, and
calcining said fine particles.